(a) If an object has a flux of 0.002 W/m^2 on your telescope, and you
    know it's 1000 kilometers (1,000,000 m) away, what's its luminosity?

(b) If an object has a flux of 0.015 W/m^2 and you know it's a big
    campfire with a luminosity of 2000 W, how far away is it?